1. Field of the Invention
This invention relates to a process for desensitization, and more specifically, to a process for desensitization in which the function of a color developer which causes a colorless color former compound to form a color is reduced or destroyed.
2. Description of the Prior Art
It has long been known to obtain a colored image by a reaction caused upon contact between an electron-donating or proton-receptive colorless organic compound (to be referred to hereinafter as a "color former") and an electron-receptive or proton-releasing solid acid (to be referred to hereinafter as a "color developer").
This phenomenon is utilized, for example, in pressure-sensitive copying sheets (for example, as disclosed in U.S. Pat. Nos. 2,505,470, 2,505,489, 2,550,471, 2,548,366, 2,712,507, 2,730,456, 2,730,457, 3,418,250, and 3,672,935), and pressure-sensitive recording sheets (for example, Japanese Patent Publication Nos. 4160/68, 7600/68 and 14039/70, and U.S. Pat. No. 2,939,009).
A printing process is also known in which an ink containing a color former is fed to a sheet coated with a color developer to obtain a colored image (see German Pat. No. 1,939,962). The color developer is as defined hereinabove, and includes, for example, clays, phenolic resins, and metal salts of aromatic carboxylic acids.
Since these color developers are generally coated uniformly on the entire surface of a support, a composition containing a desensitizer is coated using a printing press or the like on those areas of the color developer sheet which are not to be used for recording so as to desensitize those areas.
Known desensitizing agents include, for example, organic amines or quaternary salts thereof (see U.S. Pat. No. 2,777,780), and tertiary amines resulting from the chemical bonding of ethylene oxide to monoalkylamines, aralkylamines, or ethanolamine (see Japanese Patent Publication No. 35697/71), spiroacetal-type diamines and reaction products between spiroacetal-type diamines and compounds containing an oxirane group (German Patent Application (OLS) 2,343,800), and polyhydric alcohols such as polyethylene glycol or polypropylene glycol.
These desensitizing agents are all basic compounds, and react chemically with an acidic color developer to destroy or reduce the color developing activity of the color developer. However, when these desensitizing agents are used a slow drying of the printed surface occurs, and therefore, it is impossible to increase the speed of printing. When the surface coated with the desensitizer is written on or printed with a colored ink, the printed image of the colored ink markedly changes or fades, or becomes blurred.
Furthermore, when in a pressure-sensitive copying sheet a microcapsule layer containing a color former and a color developer sheet coated with a desensitizer are allowed to stand in contact with each other, the desensitizer causes the microcapsule wall to swell, and sometimes the wall membrane is destroyed. This may lead to the occurrence of fog.